Liquid toner dispersions for electrophotography are generally prepared by dispersing pigments or dyes, and natural or synthetic resins in a highly insulating, low dielectric constant carrier liquid. Charge control agents are added to aid in charging the pigment and dye particles to the requisite polarity for proper image formation on the desired substrate.
Images are photoelectrically formed on a photoconductive layer mounted on a conductive base. The layer is sensitized by electrical charging whereby electrical charges are uniformly distributed over the surface. The photoconductive layer is then exposed by projecting an image over the surface or alternatively by writing an image on the surface with a laser or L.E.D. The electrical charges on the photoconductive layer are conducted away from the areas exposed to light, with an electrostatic charge remaining in the image area. Charged pigment and/or dye particles from the liquid toner solution contact and adhere to the image areas of the plate. The image is then transferred to the desired substrate such as a carrier sheet.
One deficiency of some charge control agents is that after they have been exposed to an electric field, their conductivity can become transiently depressed. That is, upon repeated exposure to the electric field of the photoconductive layer, the ability of the charge control agent to maintain a uniform polarity diminishes over time. This is especially true in high speed electrophotographic processes wherein images must be transferred to sheets of paper running at high speed such as 100-1000 feet/min. or greater. In such systems, failure of the liquid toner charge control agents to maintain toner polarity within certain closely controlled ranges results in poor print quality. Additionally, such problems can also adversely affect mechanisms designed to feed or control toner compositions that are in some manner dependent upon toner conductivity.
Accordingly, an object of the invention is to provide a liquid toner composition having improved charge control attributes. An even more specific object is to provide improved liquid toner compositions that are capable of maintaining their polarity within a relatively narrow, uniform range even during high speed electrophotographic processes such as those mentioned hereinabove.